This invention relates to a base station control apparatus and a frequency allocation method for such an apparatus, and in particular relates to a base station control apparatus and frequency allocation method in a wireless multiplex transmission system which multiplexes a plurality of signals at the same frequency, further multiplexes a plurality of frequencies, and transmits the resulting signal.
In a code division multiplex communication system capable of using a plurality of frequencies, when an originating call is generated by a terminal, or when a terminating call is generated by the core network, the base station control apparatus compares the required power or required bandwidth (hereafter called “required resources”) for the call, and the remaining usable resources at a specific frequency predetermined in advance; if the required resources are smaller, the call is accepted, and if larger, a different frequency is allocated to the call. However, when both calls requiring large resources (high-speed calls) and calls requiring small resources (low-speed calls) coexist, conventional frequency allocation methods have no policy for frequency allocation and cannot optimally allocate frequencies, so that there is the problem that the total number of calls which can be accepted is reduced. Below, this problem is explained.
In a cellular system which provide only services for low-bitrate communications such as voice transmissions, the power required for a single call is not very great, and numerous channels can be accommodated simultaneously; hence as a result of statistical multiplexing of fluctuations in required power due to the speed of motion of individual mobile stations, fluctuations in interference power with other cells in uplink channels, fluctuations due to multipath interference in downlink channels, and other factors, the fluctuation width of reception interference power in uplink of base stations, and the fluctuation width of the total base station transmission power in downlink of base stations, are compressed (reduced).
On the other hand, in the cellular systems of recent years which provide only high-bitrate communication services, much power is required for a single call, and the number of channels which can be accommodated simultaneously is reduced, so that the effect of statistical multiplexing on fluctuations in required power due to the speed of motion of individual mobile stations, fluctuations in interference power with other cells in uplink channels, and fluctuations due to multipath interference in downlink channels, is decreased. As a consequence there are large fluctuations in the reception interference power in uplink of base stations and in the total base station transmission power in downlink of base stations.
Hence in order to ensure communication quality when providing high-speed or high-bitrate communications, large fluctuations in the reception interference power and total base station transmission power must be taken into account, by setting thresholds with an adequate margin for call connection control. FIG. 19 explains a method of traffic control for a mixture of low-speed and high-speed communications; as indicated in (A), the frequencies are RF1 to RFN, and the resource thresholds for allocation of each frequency is RTH. If the total allocated resources do not exceed the threshold, allocation to calls is possible. A terminal uses one of the frequencies from RF1 to RFN as a control channel for communication with the base station; and a frequency is set for use by the network (base station control apparatus) at the time of purchase of the terminal. This frequency is here defined as the awaiting frequency.
As shown in (B), when a high-speed call occurs for a awaiting frequency of RF1, the communication channel at frequency RF1 is already full. Hence RF1 cannot be allocated to high-speed calls, and so the next frequency RF2 is changed to the new awaiting frequency, and the communication channel at this frequency RF2 is allocated. As a result, the frequency RF2 is allocated to three high-speed calls.
In the above traffic control method, which takes the base station reception interference power and total base station transmission power as threshold values for call connections, when the types of service provided at the same frequency tend to be low-speed or low-bitrate services (low-speed calls), the effect of statistical multiplexing results in a substantial concentration effect, so that fluctuations in the total transmission power for the base station are reduced. Conversely, when high-speed or high-bitrate services (high-speed calls) tend to be more common, the effect of statistical multiplexing is not obtained and there is no concentration effect, so that fluctuations in the total transmission power for the base station tend to be large. The above concentration effect comprises both the concentration effect described in general traffic theory, and the concentration effect in the sense of statistical multiplexing of fluctuations in wireless transmission paths.
FIG. 20 explains increases in the total base station transmission power in a multiple-connection traffic model; the horizontal axis indicates the number of high-speed channels contained in one frequency, and the vertical axis is the minimum power ratio satisfying the required degradation ratio, for degradation ratios of 0.1%, 1.0%, and 10%. As the number of high-speed channels increases, the minimum power ratio increases; in particular, when the number of high-speed channels is four or greater, the minimum power ratio is seen to rise rapidly.
Hence when both low-speed and high-speed calls exist, if high-speed calls are concentrated at a particular frequency the required power at that frequency increases, and consequently the number of calls which can be transmitted at a fixed power is reduced. Because of this, when using a method of allocation to the next frequency when one frequency becomes full, as in FIG. 19, there is the problem of a possibility of concentration of high-speed calls at one frequency, so that a smaller number of calls can be accommodated. In addition, numerous fluctuations lead to difficulty in TPC (transmission power control), so that the number of calls accommodated is reduced even further.
A first technology of the prior art is a method of allocation of high-speed wireless channels to mobile stations insofar as possible (see Japanese Patent Laid-open No. 2003-125440). This first technology of the prior art is a method, when a control station is to allocate a wireless channel to a mobile station, of selecting a wireless channel enabling high-speed communication with a mobile station from among a plurality of wireless channels, and of preferentially allocating the wireless channel to a mobile station.
As a second technology of the prior art, a method of controlling the transmission rate according to the QoS has been proposed (Japanese Patent Laid-open No. 2003-179966). This second technology of the prior art provides a service which preserves fairness of service, with substantially the same QoS provided, to users of the same service class, and preserves fairness of service, with relative QoS maintained among classes, by maintaining a predetermined ratio of transmission rates among users of different classes.
The first technology of the prior art is a method of allocation of high-speed wireless channels to mobile stations insofar as possible, and does not increase the number of mobile stations (number of calls) accommodated by a base station. The second technology of the prior art provides a service which controls the transmission rate according to the Qos to preserve fairness, and similarly does not increase the number of calls accommodated by a base station.